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Laser Cleaning: Pulsed MOPA VS. CW

coating laser removal

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Laser cleaning – using a focused laser beam to quickly vaporize or strip contaminants from the surface of a material. Compared to various traditional physical or chemical cleaning methods, laser cleaning is contactless, consumable-free, non-polluting, highly accurate, and has no little damage. It is the ideal choice for a new generation of industrial cleaning technology.

Further, with high reliability, stability, and flexibility, fiber laser has become the best choice for laser cleaning beam sources. As the two major fiber lasers, continuous and pulsed fiber lasers occupy the macro and precision material processing market, respectively. For emerging laser cleaning applications, there is the question of whether a continuous laser or a pulsed laser should be used. Here we compare continuous and pulsed lasers for laser cleaning applications and analyze their respective characteristics and applicable application scenarios. We hope this article will provide a useful reference for choosing the appropriate laser cleaning technology. 

Comparison of CW fiber lasers and pulsed fiber lasers in terms of cleaning

The following are the two types of lasers used in this paper.

OperationPulsed MOPAContinuous Wave
Average Output Power (W)>200>2000
Wavelength(nm)10641080
Max.peak Power60k2000
Max. Pulse Energy (mJ)12/
Repetition Range (Hz)1-1000k10-20k
Table 1. Parameters of the test lasers

We use two types of lasers to clean different kinds of materials, one is aluminum alloy with white paint (about 20um), and the other is carbon steel with white paint (about 40um). The cleaning performance is achieved by adjusting the pulse width (100ns, 200ns, 500ns ), frequency (20-60kHz), and scanning speed (1500-9600 mm/s). The following figure shows the cleaning performance of the test laser in the table above.

cleaning performance of aluminium alloy with white paint
Fig. 1a Cleaning performance of aluminum alloy with white paint
cleaning performance of carbon steel with white paint
Fig. 1b Cleaning performance of carbon steel with white paint

Inappropriate parameters can lead to unexpected cleaning performance. For pulsed fiber lasers, lasers with lower frequencies are more likely to harm the substrate during the cleaning process. The laser with a narrower pulse width (about 100ns) can easily clean the paint. It is important to balance the heat between cleaning the paint and melting the substrate (thermal effect). Fiber pulsed lasers with MOPA structure bring the advantage of precise heat control, which is a key point in the cleaning process.

For CW fiber lasers, the slower the scanning speed, the more damage is done to the substrate. However, faster speeds will result in insufficient cleaning when the speed is above the threshold. Therefore, choosing the correct scanning speed is critical when using CW fiber lasers for laser cleaning.

 In the following article, the differences between CW fiber lasers and pulsed fiber lasers will be detailed in three main aspects: cleaning performance, cleaning efficiency, and roughness after the cleaning process.

Intuitively, materials cleaned with CW fiber lasers are darker in color compared to MOPA pulsed fiber lasers. Improper heating during the cleaning process can lead to psychological melting of the substrate, which is not acceptable in the module cleaning industry.

cleaning performance comparison on aluminium cw mopa 1 1024x571 1
Figure 2 (a) Comparison of cleaning performance of CW (left) and MOPA pulsed fiber lasers (right) on aluminum
cleaning performance comparison on carbon cw mopa 1 1024x701 1
Figure 2 (b) Comparison of cleaning performance of CW (left) and MOPA pulsed fiber lasers (right) on carbon steel

Under the microscope, the different microscopic surfaces of the cleaned substrates can be easily seen. During the CW fiber laser cleaning process, the spirit on the substrate also melts, although the paint is removed. However, when cleaning with MOPA pulsed fiber laser, the damage on the substrate is minimal, and the surface is much smoother.

microscopic surface after cleaning on aluminium cw mopa
Figure 3 (a) Microscopic surface of aluminum after cleaning with CW (left) and MOPA pulsed fiber laser (right)
microscopic surface after cleaning on carbon cw mopa
Figure 3 (b) Microscopic surface of carbon steel after cleaning with CW (left) and MOPA pulsed fiber laser (right)

In addition, Figure 4 shows the roughness Ra of the surface using CW and MOPA pulsed fiber laser. Cleaning with a MOPA fiber laser brings very little damage, and the roughness value is close to or even lower than the original surface (the laser also cleans some dust from the original surface). In contrast, when cleaning with a CW laser, the roughness value will be 1.5 times higher than the original surface. 

In addition, another advantage brought by the MOPA fiber laser is the high cleaning efficiency. When cleaning dust on aluminum alloys, the cleaning efficiency of the MOPA pulsed fiber laser is 2.77 m²/h, which is 7.7 times higher than the cleaning efficiency of the CW fiber laser (0.36 m²/h). When cleaning dust on carbon steel, the cleaning efficiency of the MOPA pulsed fiber laser is 1.06 m²/h, which is 3.5 times higher than the cleaning efficiency of the CW fiber laser (0.3 m²/h).

comparison for the roughness of the surface after cw mopa laser cleaning
Figure 4 Comparison of surface roughness after CW and MOPA pulsed fiber laser cleaning

Conclusion

Both MOPA pulsed fiber lasers and CW fiber lasers can remove dust. The cleaning efficiency of MOPA pulsed fiber laser is much faster than that of CW fiber laser at the same average output power. Also, precise heat control between cleaning and melting ensures good cleaning performance without harming the substrate.

However, the CW fiber laser is less expensive, compensating for the shortcomings in cleaning efficiency by increasing the average output power. However, it will bring worse thermal effects, which will harm the substrate.

Therefore, different cleaning applications will require different laser models. It is better to choose MOPA pulsed fiber laser for precise cleanings, such as mold cleaning. For some large steel structures, pipes, etc., the continuous laser will be a good choice due to its large size, fast heat dissipation, and low requirement for substrate damage.

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